Impedance optimized interface for membrane probe application

ABSTRACT

In a membrane probing apparatus, the impedance of the interface between coaxial cables connected to the test instrumentation and the membrane supported co-planar waveguide that conductively connects to the probe&#39;s contacts is optimized by eliminating a ground plane in the interface board.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not applicable.

BACKGROUND OF THE INVENTION

The present invention relates to probe assemblies of the type commonlyused for testing integrated circuits (ICs) that are fabricated on awafer or substrate.

The trend in electronic production, particularly in integrated circuittechnology, has been toward fabricating larger numbers of discretecircuit elements with higher operating frequencies and smallergeometries on a single substrate or “wafer.” After fabrication, thewafer is divided into a number of rectangular-shaped chips or “dies”where each die presents a rectangular or other regular arrangement ofmetallic bond or contact pads through which connections are made for theinputs and outputs of the electrical circuit on the die. Although eachdie is eventually packaged separately, for efficiency sake, testing ofthe circuits formed on the wafer is preferably performed while the diesare still joined together on the wafer. One typical procedure is tosupport the wafer on a flat stage or “chuck” and to move the wafer in X,Y and Z directions relative to the head of a probing assembly so thatcontacts on the probing assembly move relative to the surface of thewafer for consecutive engagement with the contact pads of one or more ofa plurality of dies or test structures on the wafer. Respective signal,power and ground conductors that interconnect the test instrumentationwith the contacts on the probing assembly enable each circuit on thewafer to be sequentially connected to the instrumentation and tested.

Gleason et al., U.S. Pat. No. 5,914,613, discloses a membrane probingsystem for use in a probe station. The membrane probing system comprisesa probe head and a membrane probing assembly. The probe head includes aninterface board, a multi-layer printed circuit board that facilitatesinterconnection of the membrane probing assembly and the testinstrumentation supplying power and signals to and receiving signalsfrom the electrical circuit being tested, the device-under test (DUT).The power and signals are transmitted over one or more conductors thatare conductively interconnected with respective data/signal traces onthe interface board. The data/signal traces on the interface board areconductively connected to respective conductive traces on the surface ofthe membrane assembly. A metallic layer below the surface of theinterface board provides a ground plane for the interface board and aground reference for the power and lower frequency signals.

Typically, higher frequency signals; commonly in the radio or microwavefrequency ranges, collectively referred herein to as RF signals; arecommunicated between the test instrumentation and the membrane probingsystem with coaxial cable. The coaxial cable is connected to an adapterthat is secured to the interface board. A second portion of coaxialcable, conductively interconnected with the first portion in theadapter, is connected to one or more conductive traces on the surface ofthe interface board. Typically, the end of the second portion of coaxialcable is cut at an angle and the conductors of the cable are connectedto respective traces on the interface board to transition the signalpath from the coaxial cable to a co-planar waveguide. For example, thecenter connector of the coaxial cable may soldered to a trace on theinterface board while the outer conductor of the cable, connected to aground potential, is soldered to a pair of traces that are respectivelyspaced apart to either side of the trace to which the center conductoris connected transitioning the signal path from coaxial cable to aground-signal-ground (GSG) co-planar waveguide on the interface board.The traces on the interface board are conductively engaged withrespective, corresponding traces on the lower surface of the membraneassembly extending the co-planar waveguide to the contacts on themembrane. The impedance of the transition signal path from the coaxialcable to the coplanar waveguide on the membrane is, ideally, optimized,with a typical value of 50 ohms (Ω). However, inconsistencies inconnections with the ground plane of the interface board may cause theimpedance of a particular signal path to vary from the desired matchedimpedance producing a reflection of the RF signals that are absorbed byother structures resulting in erratic performance of the probing system.

The membrane of the probing system is supported by a support elementthat is made of an incompressible material, such as a hard polymer, anddetachably affixed to the upper surface of the interface board. Thesupport element includes a forward support or plunger portion thatprotrudes though a central aperture in the interface board to projectbelow the interface board. The forward support has the shape of atruncated pyramid with a flat forward support surface. The membraneassembly which is also detachably secured to the interface board by thesupport element includes a center portion that extends over and isseparated from the forward support surface of the support element by anintervening elastomeric layer. The flexible membrane assembly comprisesone or more plies of insulating sheeting, such as polyimide film.Flexible conductive layers or strips are provided between or on theselayers to form power/data/signal traces that interconnect with thetraces on the interface board at one end. The second end of the traceson the membrane terminate in conductive connections to respectivecontacts which are arranged on the lower surface of the portion of themembrane extending over the forward support. The contacts are arrangedin a pattern suitable for contacting the bond pads of the DUT when thechuck is moved to bring the contacts of the probe assembly into pressingengagement with the bond pads.

The contacts of the probing system comprise a beam which is affixed tothe lower surface of the membrane assembly and which is conductivelyinterconnected with the appropriate trace on the surface of themembrane. A contact bump or tip for engaging a bond pad of the DUT isaffixed to one end of the beam. When the contact bump is pressed againstthe bond pad of the DUT, the membrane assembly is deflected, compressinga portion of the elastomeric layer proximate the end of the beam towhich the contact bump is affixed. The compliance of the elastomericlayer enables relative displacement of the respective contact bumps andfacilitates simultaneous engagement with a plurality of bond pads thatmay have respective contact surfaces that lie in different planes. Theresilience of the elastomeric layer controls the force exerted by thecontacts and returns the contacts to the at-rest position when the probeis withdrawn from pressing engagement with the DUT.

The bond pads on DUTs are subject to the rapid development of a layer ofoxidation which can electrically insulate the bond pad from the contact.To improve the conductivity of the bond pad/contact interface, thecontacts of membrane probes are commonly pressed into the bond pad withsufficient force to penetrate the oxide layer. While penetration of theoxide layer improves conductivity, excessive force can damage the bondpad. With the membrane probe disclosed by Gleason et al, the force ofcontact with the bond pad is exerted at one end of the beam and theoff-center loading on the beam causes the beam to rotate as the portionelastomeric layer adjacent the deflected end of the beam is compressed.The rotation of the beam causes the surface of the contact bump totranslate across the bond pad surface and abrade or scrub the oxidecoating on the surface improving conductivity between the bond pad andthe contact.

However, the conductors within the membrane assembly and attached to thecontacts can be broken by excessive displacement of the contacts or mayfail from fatigue due to repeated bending when the contacts aredisplaced during probing. In addition, bond pad material may build up inthe area of the contacts of wafer probing assemblies requiring frequentcleaning and, eventual replacement due to wear. While the membraneassembly is detachable from the interface card for cleaning orreplacement, the membrane assembly is complex and fairly expensive toreplace.

What is desired, therefore, is a probing apparatus having improvedimpedance characteristics, longer service life and less expensivecontacts that can be quickly replaced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing an exemplary membrane probingassembly bolted to a probe head and a wafer supported on a chuck insuitable position for probing by this assembly.

FIG. 2 is a bottom plan view showing various parts of the probingassembly of FIG. 1, including a support element and flexible membraneassembly, and a fragmentary view of a probe card having power, anddata/signal lines connected with corresponding lines on the membraneassembly.

FIG. 3 is a side elevation view of the membrane probing assembly of FIG.1 where a portion of the membrane assembly has been cut away to exposehidden portions of the support element.

FIG. 4 is a schematic sectional elevation view of an exemplary probingassembly.

FIG. 5 is a top plan view of an exemplary support element.

FIG. 6 is a bottom plan view of an exemplary coupon.

FIG. 7 is a sectional view of a membrane and coupon taken along line 7-7of FIG. 6.

FIG. 8 is a sectional view of a membrane and coupon taken along line 8-8of FIG. 6.

FIG. 9 is a sectional view of a membrane and coupon illustratingconnection to a conductor extending through the forward support.

FIG. 10 is a sectional view of a membrane and coupon illustrating analternative method of connecting a contact and a membrane supportedtrace.

FIG. 11 is a sectional view of a membrane and coupon illustrating analternative method of connecting a contact and a membrane supportedtrace.

FIG. 12 is a sectional view of a coupon useful with a space transformerof a needle card-type probing apparatus.

FIG. 13 is a sectional view of additional embodiments of contacts for acoupon useful with a membrane probing device.

FIG. 14 is a bottom plan view of a multi-layered tile portion of acoupon.

FIG. 15 is a sectional view of the multi-layered tile of FIG. 14 takenalong line 15-15.

FIG. 16 is a sectional view of the multi-layered tile of FIG. 14 takenalong line 16-16.

FIG. 17 is a top view of a portion of an exemplary interface board andan impedance optimized co-axial cable to co-planar waveguide interface.

FIG. 18 is a sectional view of the exemplary co-axial cable to co-planarwaveguide interface taken along line 18-18 of FIG. 17.

FIG. 19 is a top view of an alternative impedance optimized co-axialcable to co-planar waveguide interface.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring in detail to the drawings where similar parts are identifiedby like reference numerals, and, more particularly to FIGS. 1 and 4, aprobe head 40 for mounting a membrane probing assembly 42 isillustrated. In order to measure the electrical performance of theelectrical circuit or device-under-test (DUT) of a particular die area44 included on the silicon wafer 46, input and output ports of the testinstrumentation 49 are communicatively connected to contacts 81 includedon the lower portion of the membrane probing assembly and the chuck 51which supports the wafer is moved in mutually perpendicular X, Y, and Zdirections in order to bring bond pads of the DUT into pressingengagement with the probe's contacts.

The probe head 40 includes an interface board 52 on which traces 48 andshielded transmission lines are arranged for communicating data, signalsand power between the test instrumentation and the DUT. Typically, highfrequency signals are communicated between the test instrumentation andthe probe with co-axial cables 53 which connect the instrumentation toco-axial cable adapters 55 on the interface board. The shieldedtransmission lines, typically comprising second lengths of co-axialcable 50 connect the adapters to metallic traces 87 on the interfaceboard to transition the communication path from the coaxial cable to aco-planar waveguide. Referring also to FIG. 2, contact portions 86 thatterminate conductors on surface of the membrane are arranged to overlapand conductively interconnect with the traces on the interface boardextending the co-planar waveguide from the interface board to the lowersurface of the membrane assembly in the vicinity of the contacts and,eventually, to appropriate contacts 81 of the probing apparatus.

Referring also to FIG. 3, the membrane probing assembly 42 includes asupport element 54 formed of incompressible material such as atransparent, hard polymer. This element is detachably connected to theupper side of the interface board by means of four Allen screws 56 andcorresponding nuts 58 (each screw passes through a respective attachmentarm 60 of the support element, and a separate backing element 62 thatevenly distributes the clamping pressure of the screws over the entireback side of the supporting element). This detachable connection enablesdifferent membrane assemblies having different arrangements ofconductors to be quickly substituted for each other as needed forprobing different devices. A transparent window 63 enables an operatorof the probing apparatus to view the portion of the wafer in thevicinity of the probe contacts 81.

Referring to also FIG. 5, the support element 54 includes a rearwardbase portion 64 to which the attachment arms 60 are integrally joined.Also included on the support element 54 is a forward support or plunger66 that projects outwardly from the flat base portion. This forwardsupport has angled sides 68 that converge toward a flat support surface70 so as to give the forward support the shape of a truncated pyramid.Referring also to FIG. 2, a flexible membrane assembly 72 is attached tothe support after being aligned by means of alignment pins 74 includedon the base portion. This flexible membrane assembly is formed by one ormore plies of insulating sheeting such as KAPTON™ polyimide film sold byE. I. Du Pont de Nemours or other polyimide film, and flexibleconductive layers or strips are provided on the surfaces of the plies orbetween the plies to form various conductors connecting the probe'scontacts with the traces and shielded transmission lines on theinterface board.

When the support element 54 is mounted on the upper side of theinterface board 52 as shown in FIG. 3, the forward support 66 protrudesthrough a central opening 78 in the interface board so as to present thecontacts 81 which are arranged on a coupon 80 that is attached to thesurface of the membrane assembly 72 in a position suitable for pressingengagement with the pads of the DUT. Referring to FIG. 2, the membraneassembly includes radially extending arm segments 82 that are separatedby inwardly curving edges 84 that give the assembly the shape of aformee cross, and these segments extend in an inclined manner along theangled sides 68 thereby clearing any upright components surrounding thepads. A series of contact pads 86 terminate the conductors of themembrane so that when the support element is mounted, these padselectrically engage corresponding termination pads provided on the upperside of the interface board so that the traces 48 and shieldedtransmission lines 50 on the interface board are conductively connectedto the contacts 81 on the coupon 80.

The exemplary membrane probing assembly 42 is capable of probing a densearrangement of contact pads over a large number of contact cycles in amanner that ensures reliable electrical connection between the contactsand pads during each cycle despite oxide buildup on the pads. Thiscapability is a function of the construction and interconnection of thesupport element 54, the flexible membrane assembly 72 and the coupon 80.In particular, the membrane probing assembly is so constructed andconnected to the support element to enable the contacts to engage aplurality of bond pads on the DUT even if the contact surfaces of thepads are not co-planar. Moreover, the contacts on the membrane assemblypreferably wipe or scrub, in a locally controlled manner, laterallyacross the pads when brought into pressing engagement with the pads.Alternatively, the contacts may be constructed to enable the tips of thecontacts to penetrate an oxide coating on the surfaces of the pads withsubstantially vertical motion. In the event that the contacts requirereplacement due to, for examples, a built up of pad material, wear, or achange in the arrangement of the pads to be probed, the contacts can beeasily replaced by removing and replacing the coupon without the addedexpense of replacing the membrane assembly.

Referring also to FIGS. 7 and 8, for ease of illustration, an exemplaryflexible membrane assembly 72 (indicated by a bracket) comprises one ormore traces 76 affixed to an outer surface of a ply 92 comprising adielectric material which is separated by a backplane conductive layer94 from a second dielectric ply 96. However, additional dielectric pliesand conductive layers can be used in accordance with conventionallayering techniques. Preferably, the conductors of the backplane layerand the other traces are fabricated using a photolithographic process toenable precise control of the dimensions of the conductors. The membraneassembly is secured along its edges and extends over the flat supportsurface 70 of the plunger 66. Typically, the plurality of traces 76affixed to the outer surface of the membrane assembly providecommunication paths for conducting data/signals and/or power from theconductors at the interface board to the surface of the membraneproximate the forward surface of the plunger. In conjunction with thebackplane conductive layer, the conductive traces on the outer surfaceof the membrane assembly comprise controlled impedance communicationstructures connecting the interface board to the surface of the membranein the vicinity of the forward surface of the plunger. The backplaneconductive layer 94 may include portions defining one or more apertureswhich, in turn, define conductive strips of the backplane layer that arelocated proximate the various traces 76 on the outer surface of themembrane assembly. Filter capacitors 83 may be affixed to the membraneto provide frequency dependent interconnection between the conductivebackplane layer and the power and signal conductors.

FIG. 8 shows an enlarged plan view of the lower surface of an exemplarycoupon 80 that is attachable to the lower surface of the membrane 72 inarea of the forward support surface 70 of the plunger. Typically, thecoupon is attached to the surface of the membrane by an adhesive 95 thatenables removal of the coupon by breaking the adhesive bond. Theadhesive may be applied proximate the periphery of the coupon, atparticular locations on the surface or as layer that substantiallycovers the surface of the coupon but which is displaced from the pointsof electrical contact by pressure applied to the lower surface of thecoupon. The coupon includes a plurality of contacts 81 which in theillustrated exemplary embodiment are arranged in a square patternsuitable for engaging a square-like arrangement of bond pads. Referringalso to FIGS. 7 and 8, which represent sectional views taken along lines7-7 and 8-8, respectively, in FIG. 6, the coupon comprises anelastomeric layer 98 that is substantially co-extensive with the exposedsurface of the portion of the membrane that covers the forward supportsurface 70 of the plunger. The elastomeric layer can be formed by asilicon rubber compound such as ELMER'S STICK-ALL™ silicon rubber madeby the Borden Company or Sylgard 182 silicon rubber from Dow CorningCorporation. Each contact comprises a relatively thick rigid elongatebeam 100, preferably 150 to 250 microns long, having an upper surfacethat is affixed to the elastomeric layer. At one end of the beam acontact tip 102 having the general shape of a truncated pyramid isformed. For wear resistance, the contact tip may include a tip endportion 104 comprising an alloy of rhodium and nickel. Proximate the endof the beam opposite of the contact tip and on the opposite side of thebeam, a post portion 106 projects in the direction opposite of thecontact tip. The post 106 projects through the elastomeric layer and,preferably, projects proud of the upper surface of the coupon. Asillustrated in FIG. 7, the posts of selected contacts are arranged forconductive contact with one of the traces 76 on the outer surface of themembrane. As illustrated in FIG. 8, the posts of other contacts arearranged for conductive contact with contact buttons 108 projecting fromthe backplane conductive layer 94 through the lower ply 92 of themembrane assembly. Additional dielectric layers, for example layer 112,may be affixed to the lower surface of the elastic layer.

As indicated in FIG. 7, when an exemplary contact 102 is brought intopressing engagement with a respective bond pad 110, the contact force isexerted upward at the end of the beam to which the contact tip isaffixed. However, the opposite end of the beam is constrained by thecontact between the post 106 and the trace which is supported by thesubstantially incompressible membrane. The resulting off-center force onthe rigid beam 100 causes the beam to pivot or tilt against the elasticrecovery force exerted on the beam by the elastically supported lowersurface of elastomeric layer 98. This tilting motion is localized in thesense that the portion of the beam proximate the tip moves a greaterdistance toward the forward support surface 70 than a rearward portionof the same beam. The effect is such as to cause the tip of the contactto translate across the pad in a lateral scrubbing movement with asolid-line and a dashed-line representing the beginning 102 and endingpositions 102′, respectively, of the contact tip on the pad 110 which,after displacement relative to the coupon, will be in position 110′. Inthis fashion, the insulating oxide buildup on the pad is abraded so asto provide reliable probe contact-to-bond pad electrical connections. Onthe other hand, the movement of the contact does not cause flexing ofthe trace or other conductor of the membrane which substantiallyreducing fatigue of the conductors and extending the service life of themembrane.

Referring to FIG. 9, the power transmissible on the traces deposited onthe plies of the flexible membrane is limited by the cross-section ofthe traces. As a result, extremely wide traces or large numbers oftraces may be required to provide sufficient power to operate theelectrical circuits of a DUT or a plurality of DUTs. To increase thepower available for testing single or multiple DUTs and reduce thenumber and size of traces on the membrane, the probing assembly 42includes a power conductor 114 that is routed through the support 66 toa contact button 116 that extends through the lower wall of the forwardsupport surface 70. The contact button conductively engages a buss 118,which may comprise a grid, On the upper surface of the membrane 72. Atselected locations, the buss is extended downward to the lower surfaceof the membrane through apertures 119 in the other layers of themembrane. Selected contacts 81 include post portions that engage thebuss extensions to provide power to the appropriate bond pad(s) of theDUT(s). The buss can have a substantially greater cross-section than theother conductors on the membrane because it is not located in an area ofthe membrane where flexibility is required.

The resilient elastomeric layer 98 of the coupon, backed by theincompressible support surface 70 and the substantially incompressiblemembrane 72, exerts a recovery force on each tilting beam contact andthus each contact tip to maintain an adequate level of contacttip-to-bond pad pressure during scrubbing. At the same time, theelastomeric layer accommodates minor height variations between therespective contacts and pads. Thus, when a relatively shorter contact issituated between an immediately adjacent pair of relatively tallercontacts and these taller contacts are brought into engagement withtheir respective pads, deformation of the elastomeric layer enables theshorter contact to be brought into engagement with its pad after only asmall amount of further over travel by the longer contact tips.Similarly, the compressibility of the elastomeric layer enables thecontact tips to be brought into proper pressing engagement with aplurality of bond pads having surfaces that are not co-planar.

Referring to FIGS. 10 and 11, alternatively, the contacts 120 maycomprise an elongate beam portion 122 affixed to the elasticallysupported lower surface of the elastomer layer 98. The contact tipportion 102 is affixed proximate one end of the beam portion. When thetip is brought into pressing engagement with the bond pad of the DUT,the lower surface of the elastomer layer is displaced proximate the endof the beam to which the tip is affixed, but the elastic force exertedat other end of the beam causes the beam to tilt facilitating thescrubbing action when the tip is pressed toward the bond pad of the DUT.The contact 120 is conductively connected to one of the traces 76 or thebackplane layer 94 by a flexible conductive pigtail 124. The conductivepigtail may terminate on the surface of the elastomeric layer at alocation suitable for contact with a trace 76 or a contact button 108 ormaybe conductively connected with a contact pad 126 suitably located onthe surface of the coupon for contact with the appropriate trace orcontact button.

FIG. 12 illustrates a coupon 150 (indicated by a bracket) having aplurality of elastomer suspended contacts that is suitable for use withcomponents of a needle card-type probe head enabling the needlecard-type probing assembly to be converted to a lower inductance probingassembly with elastically suspended contacts by removing the needlecard-type probe head and replacing it with the coupon 150 thatinterfaces with a space transformer 152 (indicated with a bracket) ofthe needle card-type probing apparatus. In the schematic cross-sectionalview of FIG. 12, the size of certain elements and components areexaggerated, for illustrative clarity. The interface board 154 of theneedle card-type probing assembly is generally a conventional circuitboard substrate having a plurality of terminals 156 (two of many shown)disposed on a surface thereof. The terminals provide an interface forwires 158 that connect instrumentation (not shown) to the probingassembly. As illustrated, the wires may be connected to terminals on oneside of the interface board which are, in turn, connected by conductivevias 160 to terminals 162 or traces on the opposing side of theinterface board. Additional components (not shown), such as active andpassive electronic components, connectors, and the like, may be mountedon the interface board and connected to additional terminals. Theinterface board is typically round and commonly has a diameter on theorder of 12 inches. The terminals 156, 162 on the interface board areoften arranged at a 100 mil pitch or separation distance.

While some probing assemblies do not utilize an interposer, theexemplary probing assembly of FIG. 12 includes an interposer 164(indicated by a bracket) disposed between the interface board and thespace transformer. An interposer comprises interconnected electricalcontacts disposed on opposing sides of a substrate so that components onopposing sides of the substrate can be conductively interconnected. Aninterposer is often used in a probing assembly to facilitate reliableconductive connection between the terminals of an interface board andthe terminals of a space transformer. The interposer also aids inaccommodating differences in thermal expansion of the interface boardand the space transformer. The interposer 164 comprises a substrate 166and a plurality of fuzz buttons 168 (two are shown) that protrudethrough holes in the substrate. The fuzz buttons each comprise a finewire that is compressed into a small cylindrical shape to produce anelectrically conductive, elastic wire mass. As a general proposition,the fuzz buttons are arranged at a pitch which matches that of theterminals 162 of the interface board. One end of each of the conductivefuzz buttons is in contact with a terminal on the interface board whilethe second end of the fuzz button is in contact with a terminal 170 onthe space transformer. The elastic fuzz buttons are compressed providingcompliance to accommodate variations in the separation distances betweenof the various terminals of the interface board and the spacetransformer and exerting pressure on the contacts to promote goodconductivity.

The fuzz buttons protruding through the substrate of the interposer 164contact conductive terminals 170 on one side of the space transformer152. The space transformer 154 (indicated by a bracket) comprises asuitably circuited substrate 172, such as a multi-layer ceramicsubstrate having a plurality of terminals (contact areas, pads) 170 (twoof many shown) disposed on the surface adjacent to the interposer and aplurality of terminals 174 (contact areas, pads) (two of many shown)disposed on the opposing surface. In the exemplary probing assembly, thecontact pads adjacent the interposer are disposed at the pitch of theterminals of the interface board, and the contact pads 174 arranged onthe opposing surface of the space transformer are disposed at a finerpitch corresponding to the pitch and arrangement of the needle-typeprobes included in the needle card with which the space transformer wasintended to interface. While the pitch of the terminals of the interfaceboard is commonly approximately 100 mil, the pitch of needle-type probescan be as fine as approximately 125 μm. Conductive traces 176 in themultilayer substrate of the space transformer re-route the electricalconnections from the finely pitched pattern required to interface withthe probe head to the more coarsely pitched pattern that is obtainablewith a printed circuit board, such as the interface board.

The various elements of the probing assembly are stacked and anysuitable mechanism for stacking these components and ensuring reliableelectrical contacts may be employed. As illustrated, the probingassembly includes a rigid front mounting plate 180 arranged on one sideof the interface board. A stand-off 182 with a central aperture toreceive the space transformer is attached to the front mounting plate. Amounting ring 184 which is preferably made of a springy material such asphosphor bronze and which may have a pattern of springy tabs extendingtherefrom, is attachable by screws 186 to the stand-off with the spacetransformer captured between the mounting ring and the stand-off.

A coupon 150 (indicated by a bracket) comprising an elastomeric layer190 and a plurality of electrically conductive contacts is affixed tothe face of the space transformer, preferably with an adhesive 191. Thecontacts, for example exemplary contact 192, may comprise, generally, arelatively thick, elongate, rigid beam portion 194 with a post portion196 proximate one end of the beam and a contact tip 102 projecting inthe opposite direction from the opposite side of the beam proximate thesecond end of the beam. Although other shapes and materials may beutilized, the contact tip preferably has the general shape of atruncated pyramid and the distal end of the contact tip may be coatedwith a layer of nickel and/or rhodium to provide good electricalconductivity and wear resistant when the contact tip is repeatedlypressed into engagement with the bond pads of DUTs. The post 196 has arounded end distal of the beam that abuts a terminal 174 of the spacetransformer 152. The rounded end facilitates movable contact between thepost and the terminal when the contact tip is displaced upward byinteraction with a bond pad. Additional dielectric layer(s) 198 may beaffixed to the lower surface of the elastomeric layer.

The contact 200 exemplifies a second type of contact that may includedin the coupon. The contact 200 comprises a contact tip portion 202which, preferably, has the shape of a truncated pyramid or cone and abody 204 with, preferably, a square or circular cross-section. Ashoulder 206 may project from the body adjacent to the bottom surface ofthe elastomeric layer. The contact is conductively connected to theupper surface of the coupon 150 by a pig-tail 204 that is conductivelyattached to the body of the contact at one end and has a second endexposed at the upper surface of the coupon. The exposed portion of thepig-tail is arranged to contact a terminal 174 of the space transformerwhen the coupon is affixed to the surface of the transformer.Alternatively, a conductive connection between the contact and theterminal of the space transformer may incorporate a fuzz button, similarto fuzz button 168, that is embedded in the elastomeric layer with oneend exposed at the surface of the coupon and the second end abutting thecontact.

For better conductivity between a bond pad and the contact, the tip 202of the contact 200 is intended to be pushed through the oxide layer thatmay develop on the surface of a bond pad and the elasticity of theelastomeric layer may be varied to aid penetration of the oxide layer.For example, the body 204 of the contact may be extended so as todirectly contact the terminal of the space transformer, eliminating theneed for the pigtail and enabling vertical movement of the contact inresponse to pressure from the pad to be limited to the deflection of therelatively rigid space transformer while the resiliency of theelastomeric layer may enable the contact to tilt if the surface of thepad is not parallel to the end surface of the tip of the contact. On theother hand, the elastomeric layer 190 may comprise a single layer withgraduated resiliency or a plurality of sub-layers with differingresiliency to enable controlled vertical movement of the contact inresponse to the application of force at the tip of the contact. Thecoupon provides an economical way of converting a needle-type probingapparatus to a probing apparatus with much lower inductance. Therelatively long, closely spaced, needle-like probes typically exhibit asingle path inductance of 1-2 nano-Henrys (nH) which is sufficient tosubstantially distort high frequency signals and limit the usefulness ofneedle-type probes for testing devices with high frequency signals. Onthe other hand, single path inductance of 0.2 nH has been demonstratedwith elastically suspended probes of membrane-type probing apparatuses.

Referring to FIG. 13, surface penetrating contacts 252, 254 can also beutilized in a coupon 250 (indicated by a bracket) suitable for use witha membrane probing apparatus. The contact 254 may be rigidly supportedby the support 66 and the substantially incompressible membrane assembly72. As illustrated, the body of the contact is extended through theelastomeric layer 98 so that it abuts the contact button 108 of thebackplane layer 94. The abutting end of the contact is rounded enablingtilting of the contact, by compressing the elastomer surrounding thecontact's body, to aid in aligning the end of the contact with anon-co-planar pad surface.

On the other hand, the body of the contact 252 terminates in theelastomeric layer and the contact can exhibit elastic behavior asdetermined by the properties of the elastomer layer. The elastomericlayer 98 may comprise sub-layers 98A and 98B, each having differentelastic properties. The elasticity of the contact during verticaldisplacement may be determined by the properties of the sub-layer 98Awhich acts on a portion of the surface area of the upper end of thecontact and the properties of sub-layer 98B which acts on the uppersurface of a contact flange 253. A fuzz button 256 conductively connectsthe contact 252 with the trace 76.

Referring to FIGS. 14 and 15, a coupon 300 for use with a membrane probeor a space transformer may comprise a plurality of coupons or tiles 300a, 300 b, 300 c, 300 d, each independently detachably affixed to amembrane or a space transformer enabling damaged or worn portions of thecoupon to be independently replaced or portions of the coupon to includediffering arrangements of contacts 81. Flexibility in arranging contactsand conductive connections to membranes and space transformers can befacilitated by layering the conductive and non-conductive elements ofthe coupon. For example, the tile 300 c includes layered circuitryenabling conductive connection of thirteen finely pitched contact tipsto a space transformer or membrane having six coarsely pitched contactpads enabling a typically less costly, more coarsely pitched spacetransformer or membrane to interface with the finely pitched contactsuseful for probing small DUTs. The tile includes six grounded contacttips 302 including ground contact tips spaced to each side of a pair ofhigh frequency signal contact tips 304 comprising the terminations ofthe conductors of a pair of ground-signal-ground (GSG) co-planarwaveguides 310 (collectively indicated by a bracket). In addition, theexemplary tile includes a pair of contact tips 308 that are arranged toconduct DC power to a DUT and a pair of contact tips 306 that arearranged to conduct a lower frequency signal to the DUT. The contactstips of the coupon are affixed to an elastically supported lower surface314 of an elastomer layer 312. The coupon is affixed the lower surfaceto a space transformer 320 by adhesive 322.

The ground contact tips 302 are conductively connected to a conductivebackplane layer 318 deposited between a pair of dielectric layers 320and 321. The elastomeric layer is affixed to lower surface of thedielectric layer 321. The backplane layer extends over the area of thecoupon that is occupied by the contact tips 302-308. The backplane layeris conductively connected to a suitable conductor in the spacetransformer by a contact button 324 on the upper surface of the couponwhich is connected to the backplane layer by a via 326. The contactbutton 324 is arranged to engage a corresponding contact button on thelower surface of the space transformer. The contact tips 302 includepost portions 327 in contact with backplane or a pigtail 328 thatconductively connects the side of the beam portion of the contact tipand the back plane layer.

High frequency signals are transmitted between the test instrument andthe DUT through the high frequency contact tips 304. Each of the highfrequency contact tips is connected to one of a pair of contact buttons330 on the upper surface of the coupon by a via 332 and a high frequencysignal trace 324 that is deposited between the upper surface of theelastomeric layer 312 and the dielectric layer 321 which separates thehigh frequency trace from the backplane layer.

The contact tips 308 facilitating the transmission of DC power to theDUT are connected to an appropriate conductor 334 in the spacetransformer through a contact button 336 on the upper surface of thecoupon. A trace 338, including portions that are conductively affixed tothe lower surfaces of the beam portions of the power contact tips 308,is affixed to the lower surface of a dielectric layer 340 that is, inturn, affixed to the lower surface of the elastomer layer. A via 342connects the power trace on the lower surface of the coupon to a contactbutton 344 on the upper surface of the coupon.

The coupon 300 c also enables communication of a lower frequency ACsignal to the DUT through contact tips 306 which include postsconductively engaging a lower frequency signal layer 346. The lowerfrequency signal is conducted from the lower frequency signal layer tothe space transformer through a via 348 connecting the lower frequencysignal layer with a low frequency signal contact button 350 on the uppersurface of the coupon. A multilayered coupon such as the coupon 300including the tile 300 c provides flexibility for connecting anarrangement of finely pitched contact tips with the more coarselypitched contacts of more moderately priced space transformers.

Referring to FIG. 16, the coupon 300 also includes one or more formingposts 370 that extend from the upper surface of the elastomer layer 312to the lower surface of the elastomer layer. The forming posts aid incontrolling the thickness and in maintaining the planarity of thesurfaces of the elastomeric layer during the curing of the elastomer butare not conductively connected to a circuit of the probing apparatus.Posts 372, extending proud of the surfaces of the coupon are notarranged to conduct electricity but may be provided as an aid for theuser in gauging the deflection of the elastically suspended contacts.For example, a post 372 having a first end in contact with the spacetransformer and a second end projecting from the bottom of the coupon adistance slightly less than the height of a contact tip may serve as acontact gage to prevent damage to the coupon from excess deflection ofthe elastically supported contact tips. Similarly, an elasticallysupported post 374 that projects below the lower surface of the couponmay be arranged to provide a signal of the limits of contact tipdeflection. When the elastically suspended contact tip is pushed upwardby engagement with the wafer, the end of the post contacts andconductively connects traces 376, 378 in the coupon to close anelectrical circuit and provide a signal at a contact button 380 whichmay be connected to an indicator circuit.

At RF frequencies, signal frequencies in the radio frequency andmicrowave frequency ranges, it is important to match the impedance,typically 50 Ω, of the coaxial cable that typically interconnects thetest instrumentation and the probing apparatus with the impedance of thecoplanar waveguide that extends from the interface board 52 of theprobing apparatus to the lower surface of the membrane in the area ofthe forward support surface 70. A mismatch of impedance will producereflections of the signals which can couple to adjacent structurescausing frequency dependent inaccuracies in the measurements of theDUT's performance. Referring to FIGS. 4 and 17, to transition the signalpath from a coaxial cable 50 to the co-planar waveguide comprisingtraces on the membrane, the conductors of the coaxial cable are firstconductively connected to respective traces 87 on the surface of theinterface board which collectively comprise an interface board co-planarwaveguide. Typically, the center conductor of the coaxial cable isconnected to a center trace and the outer conductor of the cable, whichis connected to a ground potential, is connected a pair of side traces,one spaced to either side of the center trace to form aground-signal-ground (GSG) co-planar waveguide. The traces of theinterface board co-planar waveguide terminate proximate the edge of thecentral opening 78 in the interface board and portions of the conductivetraces on the membrane's lower surface are arranged to overlap portionsof the surfaces of respective traces of the interface board co-planarwaveguide when the membrane is clamped to the interface board. Theoverlapping surface portions conductively interconnect the traces ofinterface board waveguide to respective traces of a membrane waveguideextending the signal path to the lower surface of the membrane in theregion proximate the forward support surface 70.

Typically, the interface board includes a sub-surface conductive layerthat is connected to a ground potential to provide a ground plane forthe interface board co-planar waveguide. However, inconsistency in theinterconnection of the interface board co-planar waveguide and theground plane may produce an impedance mismatch as the signal pathtransitions from coaxial cable to co-planar waveguide. The inventorsrealized that eliminating the interconnections with the interface boardground plane would eliminate this source of error but would alsoincrease the impedance of the interface board co-planar waveguide tounacceptable levels. The inventors came to the unexpected conclusionthat the interface board ground plane could be eliminated by decreasingthe gaps between the signal trace and the ground traces of the coplanarwaveguide, interconnecting the ground traces of adjacent waveguides andproviding a low impedance structure for interconnecting the traces ofthe membrane and interface board waveguides.

The exemplary interface board 500 includes a plurality of conductivetraces deposited on the upper surface. Three traces 502, 504, 506comprise an interface board co-planar waveguide 508 (indicated by abracket). The center conductor 510 of a coaxial cable 502 isconductively interconnected with the center trace 502 of the waveguideand the outer conductor 514 of the cable is connected to a first sidetrace 504 spaced apart from one edge of the center trace and a secondside trace 506 spaced apart from the second edge of the center trace.Typically, a signal is communicated between the test instrumentation andthe probing apparatus over the center conductor of the coaxial cable andthe outer conductor is connected to a ground level electrical potential.The interface board ground plane 516, if any, terminates a substantialdistance from the area of the interface board that is overlaid by thetraces of the waveguide and is not electrically interconnected with thetraces of the waveguide. To reduce the impedance of the “ungrounded”waveguide, the width of the center trace 502 differs from the widths ofthe side traces and the widths of the respective side traces may alsodiffer from each other. For example, the center trace or signalconductor of an exemplary GSG waveguide having an impedance of 50 Ω is0.0179 inches wide and is separated from each of the spaced apart sidetraces of the waveguide by a gap of 0.004 inches. If a side trace, forexample the side trace 506 of the exemplary waveguide is adjacent to thecenter traces of two waveguides, for example center trace 502 and thecenter trace 522 of a second waveguide 520 (indicated by a bracket), theside trace is shared by the adjacent waveguides 508, 512. The side tracepreferably has a width of 0.029 inches and the outer conductors 514 oftwo adjacent coaxial cables 512, 526 are conductively connected to thetrace. If a side trace, for example the side trace 504 of the exemplaryinterface board co-planar waveguide 508, is adjacent to only one centertrace, the side trace has a width of 0.0107 inches.

Referring to also FIG. 19, while the impedance of a co-planar waveguideis substantially affected by the width of the gap between the traces ofthe waveguide, to improve the tolerance of the waveguide to misalignmentbetween overlapping portions of the traces of the membrane waveguide andthe traces of the interface board waveguide, the width of the centertrace of the interface board waveguide may be narrowed for the portionof the length that is overlapped by the membrane trace. For example, theexemplary waveguide 600 includes a central trace 602 comprising a firstlength portion 604 that is preferably 0.0179 inches wide and a secondlength portion 606 which is overlapped by the membrane trace 614 andwhich is preferably 0.0159 inches wide. The gap 608 between the centertrace and the side traces for the first length portion is 0.004 incheswhile the gap 610 for the second length portion is 0.005 inchesincreasing the tolerance to misalignment between the traces of interfaceand membrane waveguides. However, the impedance of the exemplarywaveguide 600 is substantially the same as the impedance of a waveguidewith uniform 0.004 inch gaps between traces.

The interconnection of the respective traces of the interface boardwaveguide and the corresponding traces of the membrane waveguidesubstantially effects the impedance of the waveguide. Theelectromagnetic field is carried in the gap between the traces of awaveguide and the interconnection between the traces can produce a“pinch point” that substantially increases the waveguide's impedance.The inventors concluded that the more closely the geometry of theinterconnecting structure resembles the geometry of the waveguide'straces the lower the impedance of the connection. Referring also to FIG.18, when the membrane probe is assembled, contact portions 86 of thetraces on the membrane overlap portions of the surfaces of the traces onthe interface board. The inventors concluded that a signal pathcomprising a plurality of raised pads 540 on one of the overlappingsurfaces of a pair of traces would facilitate a low impedanceinterconnection between the interface board waveguide and the membranewaveguide. The interconnecting structure for the center traces 502 and530 of the interface board and membrane waveguides, respectively,comprises four pads 540 that project from the surface of one the tracesinto contact with the surface of the other trace. A pair of the raisedpads, indicated generally as 542 and 544, is arranged substantiallyparallel to each of the edges of the center trace on which the pads areaffixed and the pads are located as close to the edge of the trace astolerance for misalignment of the trace will allow. Preferably, a raisedpad is located at each of the corners of a substantially rectangulararea on the surface of the trace.

Similarly, a portion of the side trace 534 on the membrane overlaps aportion of the side trace 504 on the surface of the interface board 500and a portion of the membrane side trace 532 overlaps a portion of theinterface board waveguide side trace 506. On the side (ground) traces,the pair 546 of raised pads 540 nearest the center trace 502 are locatedas close to the inner edge of the trace as is permitted by the toleranceto misalignment and the second pair 548 of raised pads are preferablylocated nearer the center line of the membrane trace than the secondedge of the trace. The electromagnetic field is carried in the regionbetween the traces of the waveguide and locating the pads ofinterconnecting structure closer to the region in which theelectro-magnetic field is confined reduces the inductance of theconnection.

The accuracy of measurements performed with the membrane probingapparatus, particularly at frequencies in the radio and microwavefrequency ranges, is improved by optimizing the impedance of theinterface between co-axial cables, interconnecting the testinstrumentation and the probing apparatus, and the co-planar waveguidethat extends the signal path to the contact tips. A longer service lifeand lower operating cost for a membrane probing apparatus is achievableby including the contact tips on an elastic coupon that is attachable toa membrane that includes a plurality of traces over which signals, dataand power can be transmitted. Replacing the probe needles of a needletype probing apparatus with a coupon attachable to a space transformerand including elastically supported contact tips can also improve theperformance of a needle-type probing apparatus by substantially lowerthe inductance of the apparatus.

The detailed description, above, sets forth numerous specific details toprovide a thorough understanding of the present invention. However,those skilled in the art will appreciate that the present invention maybe practiced without these specific details. In other instances, wellknown methods, procedures, components, and circuitry have not beendescribed in detail to avoid obscuring the present invention.

All the references cited herein are incorporated by reference.

The terms and expressions that have been employed in the foregoingspecification are used as terms of description and not of limitation,and there is no intention, in the use of such terms and expressions, ofexcluding equivalents of the features shown and described or portionsthereof, it being recognized that the scope of the invention is definedand limited only by the claims that follow.

1. A probing apparatus comprising: (a) an interface board; and (b) aninterface board waveguide comprising a plurality of elongate conductivetraces supported by said interface board including: (i) a center tracehaving a first edge and a second edge defining a center trace width;(ii) a first side trace spaced apart from said first edge of said centertrace and having a first side trace width; (iii) a second side tracespaced apart from said second edge of said center trace and having asecond side trace width, said second side trace width differing fromsaid center trace width.
 2. The probing apparatus of claim 1 whereinsaid first side trace width is substantially equal to said second sidetrace width.
 3. The probing apparatus of claim 1 wherein said first sidetrace width differs from said second side trace width and said centertrace width.
 4. The probing apparatus of claim 1 further comprising asecond coplanar waveguide comprising elongate, conductive tracesincluding: (a) said second side trace; (b) a second center trace havinga first edge spaced apart from second side trace and a second edge; and(c) a third side trace, spaced apart from said second edge of saidsecond center trace.
 5. The probing apparatus of claim 5 wherein saidfirst side trace and said second side trace are connected to a firstelectrical potential and said second side trace and said third sidetrace are connected to a second electrical potential.
 6. The probingapparatus of claim 5 further comprising: (a) a first coaxial cablecomprising: (i) a center conductor arranged to conduct a first signaland conductively connected to said first center trace; and (ii) acoaxial outer conductor conductively connecting said first side traceand said second side trace to an electrical potential; and (b) a secondcoaxial cable comprising: (i) a center conductor arranged to conduct asecond signal and conductively connected to said second center trace;and (ii) a coaxial outer conductor conductively interconnecting saidsecond side trace and said third side trace.
 7. A probing apparatuscomprising: (a) a first waveguide comprising a plurality of elongateconductive traces supported by an interface board, said first waveguideincluding: (i) a center trace having a first edge and a second edgedefining a center trace width; (ii) a first side trace spaced apart fromsaid first edge of said center trace and having a first side tracewidth; (iii) a second side trace spaced apart from said second edge ofsaid center trace and having a second side trace width, said second sidetrace width differing from said center trace width; and (c) a secondwaveguide comprising a plurality of elongate conductive tracesincluding: (i) a center trace including a surface portion arranged tooverlap a surface portion of said center trace of said first waveguide;(ii) a first side trace including a surface portion arranged to overlapa portion of a surface of said first side trace of said first waveguide;and (iii) a second side trace including a surface portion arranged tooverlap a portion of a surface of said second side trace of said firstwaveguide; and (d) a center trace interconnect structure conductivelyinterconnecting said overlapping surface portion of said center trace ofsaid first waveguide and said surface portion of said second waveguide;(e) a first side trace interconnect structure conductivelyinterconnecting said overlapping surface portion of said first sidetrace of said second waveguide and said surface of said first side traceof said first waveguide; and (f) a second side trace interconnectstructure conductively interconnecting said overlapping surface portionof said second side trace of said second waveguide and said surface ofsaid second side trace of said first waveguide.
 8. The probing apparatusof claim 7 wherein said center trace, said first side trace and saidsecond side trace of said second waveguide are supported by a flexiblemembrane.
 9. The probing apparatus of claim 7 wherein said first sidetrace width is substantially equal to said second side trace width. 10.The probing apparatus of claim 7 wherein said first side trace widthdiffers from said second side trace width and said center trace width.11. The probing apparatus of claim 7 wherein said center traceinterconnect structure comprises a plurality of pads projecting fromsaid overlapping surface portion of said center trace of said secondwaveguide into contact with said surface of said center trace of saidfirst waveguide, said pads including: (a) a first pad located nearer afirst edge of said center trace of said second waveguide than a secondedge of said center trace of said second waveguide; (b) a second padlocated nearer said first edge of said center trace of said secondwaveguide than said second edge of said center trace of said secondwaveguide and spaced longitudinally apart from said first pad; (c) athird pad located nearer said second edge of said center trace of saidsecond waveguide than said first edge of said center trace of saidsecond waveguide; and (d) a fourth pad located nearer said second edgeof said center trace of said second waveguide than said first edge ofsaid center trace of said second waveguide and spaced longitudinallyapart from said third pad.
 12. The probing apparatus of claim 7 whereinsaid center trace interconnect structure comprises a plurality of padsprojecting from said surface of said center trace of said firstwaveguide into contact with said overlapping surface portion of saidcenter trace of said second waveguide, said pads including: (a) a firstpad located nearer said first edge of said center trace of said firstwaveguide than said second edge of said center trace of said firstwaveguide; (b) a second pad located nearer said first edge of saidcenter trace of said first waveguide than said second edge of saidcenter trace of said first waveguide and longitudinally spaced apartfrom said first pad; (c) a third pad located nearer said second edge ofsaid center trace of said first waveguide than said first edge of saidcenter trace of said first waveguide; and (d) a fourth pad locatednearer said second edge of said center trace of said first waveguidethan said first edge of said center trace of said first waveguide andlongitudinally spaced apart from said third pad.
 13. The probingapparatus of claim 7 wherein said first side trace interconnectstructure comprises a plurality of pads projecting from said overlappingsurface portion of said first side trace of said second waveguide intocontact with said surface of said first side trace of said firstwaveguide, said pads including: (a) a first pad located nearer a firstedge of said first side trace of said second waveguide than a secondedge of said first side trace of said second waveguide, said first edgeof said first side trace being nearer said center trace of said secondwaveguide than said second edge of said first side trace; (b) a secondpad located nearer said first edge of said first side trace of saidsecond waveguide than said second edge of said first side trace of saidsecond waveguide and spaced longitudinally apart from said first pad;(c) a third pad located nearer said second edge of said first side traceof said second waveguide than said first pad; and (d) a fourth padlocated nearer said second edge of said first side trace of said secondwaveguide than said first pad and spaced longitudinally apart from saidthird pad.
 14. The probing apparatus of claim 7 wherein said first sidetrace interconnect structure comprises a plurality of pads projectingfrom said surface of said first side trace of said first waveguide intocontact with said overlapping surface portion of said first side traceof said second waveguide, said pads including: (a) a first pad locatednearer a first edge of said first side trace of said first waveguidethan a second edge of said first side trace of said first waveguide,said first edge of said first side trace being nearer said center traceof said first waveguide than said second edge of said first side trace;(b) a second pad located nearer said first edge of said first side traceof said first waveguide than said second edge of said first side traceof said first waveguide and spaced longitudinally apart from said firstpad; (c) a third pad located nearer said second edge of said first sidetrace of said first waveguide than said first pad; and (d) a fourth padlocated nearer said second edge of said first side trace of said firstwaveguide than said first pad and spaced longitudinally apart from saidthird pad.
 15. The probing apparatus of claim 7 further comprising athird waveguide comprising elongate, conductive traces including: (a)said second side trace of said first waveguide; (b) a center tracehaving a first edge spaced apart from second side trace of said firstwaveguide and a second edge; and (c) a third side trace, spaced apartfrom said second edge of said center trace of said third waveguide. 16.The probing apparatus of claim 15 further comprising: (a) a firstcoaxial cable comprising: (i) a center conductor arranged to conduct afirst signal and conductively connected to said first center trace ofsaid first waveguide; and (ii) a coaxial outer conductor conductivelyconnecting said first side trace of said first waveguide and said secondside trace side trace of said first waveguide to an electricalpotential; and (b) a second coaxial cable comprising: (i) a centerconductor arranged to conduct a second signal and conductively connectedto said center trace of said third waveguide; and (ii) a coaxial outerconductor conductively interconnecting said second side trace of saidfirst waveguide and said second side trace of said third waveguide.